I'm pretty sure the display.Interpolation property is applied to the displayed trend data after the points are aggregated and interpolated from the historian raw data to match the required point count.
These trends are both incrementing second counters. The green has an analog deadband style and the orange is discrete. Other than that they are setup the same with a deadband of 5.
The display.Interpolation is set to curveStepAfter. Setting the pointCount to a high number make the charts interpolate between samples linearly on the analog but not the discrete.
Also I just ran some tests and it looks like 8.3 doesn't function exactly as 8.1 did with the power chart and interpolation. I assume you are using 8.1 for the customer still.